Ant colonies, often described as “superorganisms,” function with remarkable coordination, akin to the cells of a single body. Each ant plays a role in maintaining the collective health of the colony. Researchers at the Institute of Science and Technology Austria (ISTA) have uncovered a fascinating aspect of this coordination: terminally ill ant brood release a specific odor to signal their impending death and the threat they pose to the colony. This discovery, published in Nature Communications, highlights a sophisticated early warning system that enables rapid detection and removal of pathogenic infections.
While many social animals attempt to hide illness to avoid social exclusion, ant pupae take the opposite approach. When faced with an incurable infection, they emit an alarm signal, alerting the colony to the contagion risk. In response, worker ants swiftly intervene, unpacking the terminally ill pupae from their cocoons and applying formic acid, an antimicrobial poison, to disinfect them. Although this treatment kills the pathogens, it also results in the pupa’s death.
Altruistic Self-Sacrifice in Ant Colonies
This self-sacrificial behavior, though seemingly detrimental to the individual, benefits the colony. “What appears to be self-sacrifice at first glance is, in fact, also beneficial to the signaler: it safeguards its nestmates, with whom it shares many genes,” explains Erika Dawson, the study’s first author and former postdoc in the Social Immunity research group at ISTA. By warning the colony, terminally ill ants help maintain its health, ensuring the continuation of their genetic lineage through future generations.
The study, conducted in collaboration with chemical ecologist Thomas Schmitt from the University of Würzburg, Germany, is the first to describe this altruistic disease signaling in social insects. If a fatally ill ant were to hide its symptoms and die unnoticed, it could become a highly infectious threat to the entire colony. Instead, active signaling allows for effective disease detection and pathogen removal.
The “Find-Me and Eat-Me” Signal
In the complex social structure of an ant colony, individuals function much like cells in a body. While reproductive queens produce offspring, non-fertile workers handle maintenance and health tasks, mirroring the specialization seen in human cells. This cooperation is crucial for the survival of the colony, with individual ants sometimes sacrificing themselves for the greater good.
But why evolve such a complex early warning system when sick animals can simply isolate themselves? “Adult ants that approach death leave the nest to die outside the colony. Similarly, workers exposed to fungal spores practice social distancing,” explains Sylvia Cremer, head of the research group. However, immobile ant brood, like infected cells in tissue, cannot isolate themselves and must rely on external assistance.
Both body cells and ant brood emit a chemical signal to attract helpers—immune cells or worker ants—to detect and eliminate them as infection sources. Immunologists refer to this as the “find-me and eat-me signal.” According to Cremer, “The signal must be both sensitive and specific, identifying all terminally-sick ant pupae while avoiding the unpacking of healthy ones.”
Changes in Pupal Scent Profile
Schmitt’s research focuses on chemical communication in social insects. He notes that workers specifically target individual pupae, indicating that the scent must be directly associated with the diseased pupa. The signal comprises non-volatile compounds on the pupal body surface, with increased intensity of two odor components signaling terminal illness.
To test this, researchers transferred the signal odor to healthy pupae, observing the workers’ reaction. “We extracted the smell from the signaling pupae and applied it to healthy brood,” Cremer describes. The results were conclusive: the altered body odor alone prompted the ants to unpack the pupae, confirming its role as a ‘find-me and eat-me’ signal.
Signaling Only in Uncontrollable Cases
Interestingly, ants do not signal infection indiscriminately. “Queen pupae, with stronger immune defenses, do not emit this warning signal,” Dawson explains. “Worker brood, unable to control the infection, signal to alert the colony.” By signaling only when an infection is uncontrollable, the sick brood enable the colony to respond proactively to genuine threats, ensuring that individuals capable of recovery are not sacrificed unnecessarily.
This precise coordination between individual and colony levels makes altruistic disease signaling effective. As Cremer concludes, “This approach ensures the colony remains healthy while preserving individuals that can recover.”
The study underscores the complexity and sophistication of ant colony dynamics, providing insights into how social organisms manage disease threats. Such research not only enhances our understanding of ants but also sheds light on broader biological processes.
In the pursuit of understanding fundamental biological processes, animal studies remain indispensable. Despite advancements, alternative methods like in silico models cannot fully replace the insights gained from studying living organisms. These studies are conducted under strict legal regulations to ensure ethical treatment of animals.
